Single- and double-stranded DNA: building a trigger of ATR-mediated DNA damage response.

The DNA damage signaling pathways mediated by the ataxia-telangiectasia mutated (ATM) and the ATM and Rad3-related (ATR) kinases play crucial roles in the maintenance of genomic integrity and may function as an anti-cancer barrier during early tumorigenesis. Although the ATM and ATR pathways share some of their downstream functions, the DNA damage that evoke these two pathways are distinct. While ATM plays a primary role in the response to double-stranded DNA breaks (DSBs), ATR controls the response to a much broader spectrum of DNA damage, including many that interfere with DNA replication. And, unlike ATM, ATR is crucial for maintaining genomic integrity during S phase of the cell cycle, and is indispensable for cell survival. Clearly, revealing the DNA structure that elicits the ATR pathway would be a critical step toward understanding the essential function of ATR and the genomic instability that it counters. The versatility of the ATR pathway in DNA damage response suggests that this pathway is likely able to sense a common signal generated by different types of DNA damage and genomic instability. Two simple structures commonly generated at sites of DNA repair and stressed DNA replication forks are single-stranded DNA coated with replication protein A (RPA-ssDNA) and junctions of singleand doublestranded DNA. Both of these structures have been implicated in the activation of ATR checkpoint by a number of studies using different model organisms. In this issue of Genes & Development, Cimprich and colleagues (MacDougall et al. 2007) report that circular singlestranded DNA (ssDNA) annealed with primers specifically triggers the ATR-mediated checkpoint responses in Xenopus egg extracts, revealing the first defined DNA structure sufficient to activate the ATR checkpoint pathway.